samples: bluetooth: hci_uart: Fix command len calculation

Due to a historical error introduced during an hci_uart refactor, the
get_len() function was using the wrong HCI H4 packet identifier for
commands. Instead of using H4_CMD (0x01) it was trying to match
BT_BUF_CMD (0x00).

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>